With the development of oil and gas exploration towards deeper and harder strata, the requirement for drilling is also getting higher and higher, and more and more attention has been paid to increasing the rate of penetration (ROP). It is the most effective method to increase ROP by releasing rock stress in advance which changes a stress field of a drilled stratum. At present, domestic and foreign experts and scholars implement this method in two ways. The first way is to release the stratum stress based on hydraulic energy of a drilling fluid, which is represented by a high-pressure pulsed jet, a cavitation pulsed jet, an abrasive pulsed jet and the like. The way of releasing the stratum stress based on the hydraulic energy requires an addition of a special tool mounted above the drill bit, which, to a certain extent, increases consumption of drilling fluid energy. At the same time, it also needs to take into account the safety and reliability of the special tools mounted. The second way is to change stress field during rock breaking by changing a contact shape between the drill bit and the bottom hole, which is represented by Kymera combined bit and a reaming while drilling bit. By the way of changing the contact shape between the drill bit and the bottom hole which releases the stress of the drilled formation, the purpose of reducing the stress of the formation at the bottom hole can be achieved just by optimizing design of the drill bit, and it has a broad development prospect. However, during the process of drilling into interlayers by the Kymera combined bit and the reaming while drilling bit, a pilot bit is prone to collapse or increased wear and so on due to their inability to automatically adjust the distribution of the weight on bit (WOB), resulting in reduced service life, and therefore the best speed-increasing effect of such drill bit cannot been achieved.
Chinese invention patent with publication No. CN105443041A discloses a weight-on-bit self-adjusting drill bit, and specifically discloses the following features: the weight-on-bit self-adjusting drill bit comprises a joint, a fluid-passage and force-transmission short connector, a reaming bit, a force storage spring, and a pilot bit; the joint, the fluid-passage and force-transmission short connector and a pilot bit are connected into a whole through thread; the reaming bit is located between the joint and the pilot bit, and mounted outside the fluid-passage and force-transmission short connector; an axial relative movement between the reaming bit and the whole can be realized, where said whole is comprised of the reaming bit and the joint, the fluid-passage and force-transmission short connecter and the pilot bit; the force storage spring is mounted in the middle of the fluid-passage and force-transmission short connector, which configured to reserve elastic force and to recover relative position between the reaming bit and the whole. The invention has the advantages of simple principle and structure, and wide application range, and can be configured to ensure fast drilling with small WOB when performing deviation-controlled straight drilling and improve the drilling efficiency of vertical wells in an easily inclined formation. The drilling process is completely the same as conventional drilling, which is conducive to popularization and use.
Chinese invention patent with publication No. CN105317377A discloses a central weight-on-bit self-adjusting drill bit, and specifically discloses the following features: the central weight-on-bit self-adjusting drill bit comprises a reaming bit, a weight adjusting spring, a shunting and force-transmission assembly and a pilot bit; the pilot bit and the shunting and force-transmission assembly are connected into one body which is mounted in an axial torque transmission hole of the reaming bit, where the pilot bit protrudes out of a crown of the reaming bit; the weight adjusting spring is arranged between a top of the shunting and force-transmission assembly and a lower end of a joint of the reaming bit; the reaming bit can be disposed into one or more stages. The invention has the advantages of simple principle and structure, wide application scope, and it can be applied to all kinds of stratum with better using effect in the interbedded stratum; it can be used with the pulse jet generator to further improve the ROP; in the course of drilling, the operation is identical to that of conventional drilling, and there is no special requirement for ground facilities, a drilling string, a bit type, which is advantageous to the drill bit popularization and the filed application.
The weight-on-bit self-adjusting drill bit disclosed in the above two patent applications use the pilot bit which bears partial WOB to break the stratum and drill a small diameter wellbore in advance, and the reaming bit, which bears another part of the WOB, to ream the drilled small-diameter wellbore. By using the spring between the pilot bit and the reaming bit, the WOB distribution on the pilot bit and the reaming bit is adjusted to be good, and the relative ROP between them is controlled to be reasonable, thereby improving the ROP. An indoor test verifies that the ROP can be improved by this kind of drill bit, especially in the multi-interlayer stratum. However, the effect of this kind of drill bit has not yet achieved the best, and the ROP is expected to increase further if other speed-increasing schemes can be implemented on such drill bit.